Sampling Tools for Physical Capture

Essential information about fish is traditionally acquired through physical capture. For a fish stock, derived information of particular interest is the geographical distribution and abundance, size distribution or so-called stock structure, rates of growth and sexual maturation, survivability or mortality due to fishing and to other physical causes, and stomach contents, as indicators of diet composition and availability of prey. Recruitment, or the quantity of fish entering the spawning part of the stock for the first time, may be measured or assessed through such information. Species diversity can also be gauged by observing the range of species that are caught, together with their corresponding numerical densities.

When physical capture is performed by research vessels, special gear can be used, and both gear and procedures can be standardized. This allows a much higher degree of control than could be routinely obtained from fishing vessels participating in a directed fishery. This applies to such parameters as size of vessel, tow speed, tow location, and size of net, among other things. Data on taxonomy, species distribution, abundance and biomass, age structure, fecundity, diet composition, and maturity, can then be used in the conduct of science, as well as for management and regulatory purposes. In particular, the state of fish stocks and efficacy of possible management measures already in place can be assessed. The need for changes in management measures may be indicated or suggested through the same fishery-independent data. These data are also used to verify trends observed from data collected from fishing vessels participating in various fisheries.

In the following sections, some methods of catching are briefly summarized. These include methods currently being used by the NOAA National Marine Fisheries Service (NMFS) Northeast Fisheries Science Center (NEFSC) in Woods Hole, Massachusetts, and by the Department of Fisheries and Oceans (DFO), Canada.

All methods of physical capture are inherently selective. Small fish may pass through large-meshed nets; large fish may out-swim trawls; gill nets will catch fish mainly of a certain size range. Fish may react differently to fishing gear with respect to species, size, biological state, environmental conditions including ambient light and the acoustic noise field, among many other factors (Walsh et al. 1993, Godø 1998). This is why fish behavior is studied assiduously by designers of fishing gear, especially for research use.

A bottom trawl is a set of gear that enables a net to be dragged along the bottom to catch fish that are on or near to the bottom, typically referred to as demersal fish or groundfish. The gear is generally rather complicated, having evolved through an extensive process of trial and error to accomplish a task that is intrinsically difficult and hazardous, often with the additional aim of improving the representativity of the catch.

Components of one kind of bottom trawl, an otter trawl, are indicated in Fig. 1. Principal parts are identified. The cod-end is the fine-meshed net most distant from the towing vessel that retains fish and other organisms exceeding the mesh size. In contrast to commercial fishery operations, which generally use larger-mesh gear to capture commercially-marketable fish, research gear often uses smaller mesh to intentionally capture small fish to determine the number of fish available to fisheries in future years. The body of the net is relatively coarse-meshed and usually very large, filtering a huge volume of water during its towing (Fig. 2). The net is held open by a combination of floats attached to the headrope of the net and large metal vanes called doors or otters (Fig. 3). By pulling these through the water by means of attached wires, called warps, the hydrodynamic force keeps the net spread open in the horizontal plane. The lower opening of the net is attached to a footrope, which passes along the bottom by means of rollers or bobbins [Yankee 36 Bottom Trawl, NEFSC Gear].

Another variant of the bottom trawl is the so-called beam trawl, illustrated in Fig. 4. The principal difference with the otter trawl is the use of a fixed bar or pole – the beam – to maintain a constant horizontal spread at the opening.

The otter trawl is the preferred bottom trawl in the Gulf of Maine. The beam trawl is more often used in European waters, e.g., the North Sea.

A particular kind of otter trawl is used in the long-running Bottom Trawl Survey of the Northeast Fisheries Science Center (NEFSC) [Anthony 1996]. This is the Yankee 36 roller trawl [Spring and Autumn Bottom Trawl, NEFSC Surveys], which has been used since the inception of the annual autumn survey in 1963 and annual spring survey in 1968 [Smith 2002]. During the period 1972-1981, the Yankee 41 trawl was used in an attempt to achieve better sampling of pelagic fish higher off the bottom than the usual targeted groundfish species. During the winter surveys, begun in 1992, a so-called "flat-net" trawl has been used to obtain more representative samples of demersal fish species, including flounders, skates, and goosefish [Winter Trawl Survey, NEFSC Surveys].

Because of the use of the several bottom trawls in scientific work with applications to management, standardization has been considered to be essential. Not only is the gear standardized, but so is its use, with fixed tow duration of 30 minutes at a constant speed over ground of 3.8 knots. Standardized otter trawls are also in research use by the Department of Fisheries and Oceans (DFO), Canada, but with generally different towing speeds and durations. These have included the Yankee 36, Engel 145 High-Opening, and Atlantic Western IIA groundfish otter trawls for use in the Scotia-Fundy region (Carrothers 1988). The Western IIA box otter trawl has been used to catch cod on the northern Grand Banks by the R/V Alfred Needler (Morgan et al. 1997). The Campelen 1800 shrimp trawl is also used to capture bottom fish in the Newfoundland region.

Figure 17. Photo of the MOCNESS as it is being lowered into the water for towing behind the ship. Credit: Photo by Lew Incze. Enlarge

Figure 18. Schematic diagram of MOCNESS during operation with one net open. Credit: Gulf of Maine Research Institute: http://www.gmri.org/Enlarge

A midwater trawl is a set of gear that is used to catch fish that are between the sea surface and bottom, generally staying clear of the bottom. Occasionally, midwater trawls are configured with floats to perform catching in the shallow-surface layer.

Midwater and bottom trawls have many parts in common, if differing in dimensions and shapes due to their different fishing objects and hydrodynamic regimes of operation. Midwater trawls are designed to catch fish in the midwater column, hence must be capable of rapid maneuvering while maintaining an open net mouth. This is reflected in differences in the body of the net, rigging, and even trawl doors.

A particular midwater trawl is shown in Fig. 5. This is the High-Speed Midwater Rope Trawl (HSMRT). This custom-designed trawl is used by NEFSC to provide essential ground-truthing data on species and size composition during the hydroacoustic surveys of Atlantic herring (Fig. 6) [Operations, Pelagic Trawling, NEFSC Fisheries Acoustics Research]. A profile of the path of the HSMRT during a particular tow is shown in Fig. 7.

Shrimp trawl

A special small-mesh bottom trawl is used to catch northern shrimp in the Gulf of Maine. This follows the basic design of the otter trawl shown above in Fig. 1, but with modified shape and dimensions, (Fig. 8) [Shrimp Net, NEFSC Gear]. The groundrope and sweep are configured to optimize the capture of shrimp. The Portuguese polyvalent otter doors weigh 350 kg, 100 kg less than the doors used for NEFSC Bottom Trawl Surveys [Trawl Doors, NEFSC Gear].

Sea scallop dredge

An eight-foot-wide sea scallop dredge is shown in Fig. 9 [Scallop Dredge, NEFSC Gear]. This is dragged along the bottom at depths as great as 120 m (Fig. 10). During scientific surveys conducted by NEFSC, a constant dredge haul duration of 15 minutes and a towing speed of 3.8 knots is maintained.

While the sea scallop dredge was designed for the mentioned shellfish, it also catches flounders, hakes, goosefish, rocks, shells, and invertebrates in significant quantities (Fig. 11).

The first NEFSC sea scallop dredge survey was conducted in 1975. Since 1977, the survey has been performed annually during the summer [NEFSC Surveys].

Hydrodynamic clam dredge

A hydrodynamic clam dredge is an immensely complicated piece of fishing gear, as suggested in Fig. 12 [Clam Dredge, NEFSC Gear]. A cage design clam dredge uses a submersible, electrically-powered pump to jet water into the seabed to dislodge clams. The dredge is deployed using a metal deployment cable, but towed with a flexible towing hawser. During the scientific surveys conducted by NEFSC, a constant dredge haul duration of 5 minutes and a towing speed of 1.5 knots are maintained. It is an effective gear for sampling both the surf clam and ocean quahog, but also catches other benthic fauna, e.g., invertebrates.

NEFSC conducts triennial surveys with this gear. It enumerates both clams and quahogs, as well as certain other invertebrates. The primary survey areas extend from Cape Hatteras to Georges Bank. Some additional surveying has been done in Massachusetts Bay to evaluate fishery potential for the Arctic surf clam (Mactromeris polynyma) and off the Maine coast to evaluate populations of ocean quahog [ NEFSC Surveys].

Plankton nets

A large number of nets have been devised for catching plankton. The subject has been comprehensively reviewed by Sameoto et al. [2000] and by Wiebe and Benfield [2003]. Here, three kinds of plankton nets are illustrated.

Ring net

This simple net consists of a fine-meshed bag attached at its mouth, or opening, to a metallic ring (Fig. 13). The net itself is terminated in a bottle or jar where the unfilterered plankton and other particulate matter are collected (Fig. 14).

The net is usually deployed vertically for non-quantitative purposes from a platform, such as a vessel or pier. It may also be towed, although lacking in devices for controlling its passage through the water column, which is otherwise determined by hydrodynamic forces generated naturally during towing or hauling. Towing applications are mainly non-quantitative.

Bongo nets

Floating or suspended fish eggs and newly hatched larvae are often caught with Bongo nets, as illustrated in Figs. 15 and 16 [NEFSC Surveys]. The name is due to the perceived resemblance to the eponymous paired drums. The mesh size is very fine, ranging from 20 µm up to 1000 µm (1 mm), thus allowing eggs and larvae with sizes of order 1-20 mm to be caught. The nets, mounted on a rigid yoke, can be towed from the surface to near the bottom for sampling throughout the water column.

In order to obtain quantitative samples of phytoplankton, zooplankton, other invertebrates, and large fish, it is critical to estimate the volume of water that is filtered during the sample. Most bongo and ring nets are deployed with mechanical or electronic flow meters positioned in the mouth of the net to quantify the volume of water filtered.

The Multiple Opening/Closing Nets and Environmental Sampling System (Fig. 17), which is generally known by its acronym MOCNESS, is an operational, widely used system for capturing plankton at specific depths on the command of the operator. It also routinely carries a number of sensors for measuring environmental parameters as it is towed. These sensors measure, for example, conductivity, temperature, pressure, fluorescence, optical transmission, dissolved oxygen, and light levels.

As a system for capturing zooplankton, MOCNESS in its current configuration consists of a number of nets. These are designed for towing at a 45 degree angle. They can be opened and closed on command from the towing vessel. Originally, the system carried a total of nine nets. The mesh size of the original nylon nets, each 6-m long, was 333 µm. The opening of the original nets was rectangular, 100×141 cm, with the dimensions chosen so that the projected area of the system as oriented during trawling relative to the tow direction would be 1 m2 (Fig. 18). Basic alternate models of MOCNESS use sets of identical nets with one of these mouth openings: 0.25, 1, 2, 4, 10, and 20 m2. Each net is terminated with a bottle or jar that holds the catch.

The cable used for towing MOCNESS is electrically conducting. This enables power and commands to be sent from the towing vessel to the system, and environmental data from the on-board sensors to be transmitted back to the towing vessel in the same way.

In addition to its wide use in oceanographic research, MOCNESS has provided inspiration for other developments, as indeed it has been inspired by its predecessors. One notable recent development is a multiple opening and closing pelagic trawl system designed and built by the Institute of Marine Research, Bergen. This system, called MultiSampler, enables pelagic fish to be caught at depths selected by the operator during deployment, similarly to the way that capture depths are selected by the operator of MOCNESS.

Acknowledgments

Dr. Russell Brown, Branch Chief, Ecosystems Surveys, NEFSC, is thanked for discussion and review of this section. Dr. Joseph DeAlteris, Professor of Fisheries and Aquaculture, University of Rhode Island, is thanked for the use of illustrations, as is Aquatic Research Instruments.